The causes of neurodegenerative disorders such as Huntington's disease, Parkinson's disease and Alzheimer's disease are unknown. These disorders are characterized by selective vulnerability of discrete neuronal populations, the basis for which is not understood. "Excitotoxicity" at the NMDA receptor has been implicated in each of these diseases, but NMDA receptor distribution alone does not predict where pathology will occur. There is preliminary evidence suggesting distinct agonist-preferring and antagonist-preferring NMDA receptors. It might be hypothesized that regions of brain could be rendered selectively vulnerable by having high concentrations of agonist-preferring NMDA receptors; other, less vulnerable regions that have a high density of NMDA receptors might have more antagonist-preferring receptors. Based on the hypothesis that there are subtypes of NMDA receptors, experiments have been designed to look for distinct agonist- and antagonist-preferring receptors, as well as receptors that are differentially regulated by glycine. Autoradiographic receptor binding will be used to compare directly the characteristics and distributions of NMDA agonist and antagonist binding. Binding studies will also be used to determine whether there are regional differences in glycine modulation of the NMDA receptor ion channel. Toxicity at the NMDA receptor is modulated by neuronal energy status, and the mitochondrial respiratory chain is the ultimate source of neuronal energy. Thus, the regional distribution of the electron transport system could be a determinant of the regional and cellular distributions of the respiratory chain enzymes in the brain. Novel methods are proposed for localizing two of the respiratory chain enzymes (complex I and complex IV) in brain with a very high degree of anatomical resolution. First, [3H]dihydrorotenone binding will be used as a specific, high affinity probe of complex I in an autoradiographic assay. Second, a fluorescein moiety will be linked to rotenone and binding of this compound will be used to localize complex I at the cellular level with fluorescence microscopy. Finally, immunocytochemical and histochemical techniques will be used to localize complex IV. Studies of NMDA receptor subtypes and electron transport complexes will then be extended to animal models of neurological disease and to human post-mortem brain specimens. The proposed studies should provide new insights into NMDA receptor pharmacology, regional mitochondrial respiration in brain and, ultimately, mechanisms of selective vulnerability in neurodegenerative diseases.